"I was trying on a bathing suit, and I actually just sent that to my boyfriend, my fiancé, and I was like, 'Babe, can I post this up? This is my big, like, middle finger to the world on everyone that called me fat,' " Kardashian, 33, told Jay Leno.

The criticism of her weight was excruciating, she adds, and drove her to seek happiness out of the spotlight for a while.

"It really hurt my soul," she said. "I was gaining weight because I was pregnant, and it was just really hard to read all these stories and hear all these nasty things … Why would I subject myself to so much negativity, especially now that I have a daughter? … I've definitely tried to live more of a private life because of that."

Getting her body back after the birth of baby North was the "hardest challenge of my life," she said, but she has managed to lose 50 lbs. thanks to exercise and the Atkins diet.

On the topic of their wedding, Kardashian said she and Kanye West are "hoping for next summer" and will probably pick "somewhere destinational" – perhaps Paris or Italy. "Maybe I'll pick the place and then he can plan it," she said.